1. Field of the Invention
The present invention relates to a density control method for image forming apparatuses, and an image forming apparatus that is capable of executing the density control method.
2. Description of the Related Art
In conventional image forming apparatuses of an electrophotographic type such as copying machines, printers, and facsimiles, an electrostatic latent image is formed on a photosensitive member by exposure, is developed using toners, and is transferred onto a transfer sheet (recording sheet) and then visualized and fixed to form an image on the transfer sheet. Such electrophotographic image forming apparatuses, which form a color image using toners of four colors, i.e. yellow, magenta, cyan, and black, have become widely used.
There are two types of electrophotographic printers for forming a color image: a one-drum type in which toner images of respective colors are successively formed/transferred on one photosensitive member, and a four-drum type in which toner images formed by respective four exposure units, respective four developing devices, and respective four transfer devices are successively transferred onto a transfer sheet to form a color image thereon. In the four-drum type, an image can be transferred onto a transfer sheet on one path, and hence the throughput of color images can be increased as compared with the one-drum type.
For both the one-drum type and the four-drum type, there have been known a direct transfer method in which a toner image is directly transferred from a photosensitive member onto a transfer sheet, and an indirect transfer method in which a toner image is transferred from a photosensitive member onto an intermediate transfer member such as an intermediate transfer belt and is then transferred onto a transfer sheet. Generally, as compared with the direct transfer method, the indirect transfer method is less affected by surface shape, moisture content, etc. of a transfer sheet and hence enables more stable images to be obtained.
However, as photosensitive member surface characteristics, toner characteristics, transfer characteristics, and so forth vary according to environmental change and durability deterioration, resulting in variations in toner density, so that images are affected by such variations in toner density as well as by variations in quality of transfer sheets. For this reason, to obtain more stable images, it is necessary to reduce variations in toner density whether the one-drum type or the four-drum type is used. Particularly in the case where a color image is formed, variations in toner density between toners of respective colors are likely to cause a tinge in the formed image.
Conventionally, to correct for variations in toner density, there have been proposed, for example, a method in which a maximum density correction control process is carried out such that the density of a density patch with the maximum density is measured to detect latent image characteristics and development characteristics of the surface of a photosensitive member so that image forming conditions can be adjusted according to the characteristics, and a method in which a density gradation correction control process is carried out such that the densities of a plurality of density patches corresponding to a plurality of density gradations and formed for respective colors are measured to create a data conversion table for correcting data corresponding to each density information item according to detection results when creating actual image data (refer to the specification of U.S. Pat. No. 5,752,126).
According to the maximum density correction control process as described above, the image forming conditions of the image forming section are changed to correct variations in density reproduction range between respective colors. According to the density gradation correction control process, image data is corrected to correct for variations between density curves of density gradations of respective colors.
On the other hand, an increasing number of image forming apparatuses having a plurality of image forming speeds have been used so that color images can be formed on transfer sheets made of various materials mainly due to the necessity of decreasing the fixing speed in terms of the fixability, etc. of thick sheets, OHP sheets, and other types of sheets.
Specifically, if the one-drum type is used, in either the direct transfer method or the indirect transfer method, an image is formed at the same speed on all kinds of materials at stages up to a transfer section, and the sheet conveying speed is decreased at the stage of fixing. Therefore, it is unnecessary to have a plurality of image forming speeds.
If the four-drum type is used, however, in either the direct transfer method or the indirect transfer method, the conveying speed of a transfer sheet and the image forming speed must be equal to each other since transfer of toner images of respective colors is carried out on one path. If an image transfer section which transfers images onto a transfer sheet is sufficiently remote from a fixing section, the conveying speed of the transfer sheet and the image forming speed can be different, but in this case, the image forming apparatus has to be increased in size. For this reason, an increasing number of image forming apparatuses having a plurality of image forming speeds have been used.
In the above described image forming apparatus having a plurality of image forming speeds, in the case where densities of images formed at respective image forming speeds are required to be made uniform, the maximum density correction control process and the density gradation correction control process as described above must be carried out each time the image forming speed is changed, for the reason that as the image forming speed is changed, the amount of exposure per unit area during formation of an electrostatic latent image varies in terms of an integral component, and the amount of toner supplemented per unit area during development also varies.
However, if the maximum density correction control process and the density gradation correction control process are carried out at all image forming speeds, a period of time required for control raises a problem.
Specifically, in either the maximum density correction control process or the density gradation correction control process, an image forming sequence comprised of electrostatic charging, latent image formation, development, and intermediate transfer must be carried out to form a toner density patch and measure the density thereof. For example, if three image forming speeds are provided, correction control corresponding to the respective image forming speeds must be provided each time the image forming speed is changed, and thus, if the image forming speed is changed twice, a triple or more period of time for control is required